Proposed investigations will fall into three general categories: 1. The degradation and rejoining of ruptures in the DNA structures of Chinese hamster ovary (CHO) cells will be explored at progressively smaller X-ray doses below 1000 rads. In this way our investigations will be extended into the region of truly biological doses. 2. The relationship between chromosomal aberrations and the cellular survival of the very radiosensitive L5178Y murine leukemic lymphoblast line will be investigated as a function of cell cycle position. 3. Because of the ever-increasing potential importance of hyperthermia in radiation therapy and the forthcoming Hyperthermia Conference, we have been asked by Dr. W. C. Dewey to devote time to the study of DNA damage and repair in CHO cells at elevated temperatures.